Field of the Invention
The present invention relates to a waveguide for the extraction of light at low levels of reflection.
Description of the Related Art
For certain electro-optical components with waveguides, such as lasers, amplifiers and modulators, it is necessary to achieve very low levels of reflection at the point or points at which the light is extracted from the waveguide after a light-generating crystal. The reflection, seen from inside of the crystal, is typically approximately 30% for a waveguide of, for example, indium phosphide that is abruptly interrupted. Levels of reflection that lie below 0.001% are required for certain applications.
The coating of the sectioned exit surface with dielectrics whose thickness and refractive index are adapted to give low levels of reflection is known. Additional measures, however, are required. If the angle between the waveguide and the section plane is not 90 degrees, only a small part of the light is reflected back into the waveguide. The remainder is reflected into different directions and is lost.
It is, of course, possible for the waveguide to be perpendicular to the facet of the component over a part of the length of the component and then to be bent such that the angle it makes with the facet is no longer a right angle. This is possible in, for example, an SOA (semiconductor optical amplifier).
Another method is the use of a window construction, which involves the waveguide being terminated within the component, the chip, before the waveguide reaches the facet. In this way, the light is spread by diffraction before it reaches the facet. Due to the spreading of the light beam, only a small part of the light reaches back into the waveguide after reflection in the facet. The region of the window, i.e. the region between the end of the waveguide and the facet of the component, may have a length of, for example, 5-50 micrometers, for InP components. The light spreads in this manner in a lobe with an angle of a few degrees.
Even lower levels of reflection can be obtained by combining oblique placement of the waveguide, such that it is not orthogonal with the facet, and a window region. If the obliqueness of the waveguide is greater than the width of the diffracted lobe, the fraction of light that reaches back to the waveguide will be very small.
Placement of the waveguide at a very high obliqueness causes problems, since the chip in this case will be wider. Other problems of a process technical nature may also arise such that anisotropic etching, for example, may give a disadvantageous form of mesas in the direction of obliqueness.